1. Field of the Invention
The present invention relates in general to the field of telecommunications, and more particularly, to an improved bi-directional buffer line amplifier for use with a U-interface Basic Rate ISDN (Integrated Services Digital Network).
2. Description of the Prior Art
Modern digital telecommunication systems have operating requirements that interface their digital operation to an analog transmission environment. Many such systems use a Digital Network Interface Circuit (DNIC) device to interface a digitally operating system to an analog transmission line. The DNIC is capable of providing high speed, full duplex digital transmission up to 160 kbit/s over a standard analog twisted wire pair. Further, the DNIC uses adaptive echo-cancelling techniques that transfers data in a (2B+D) format compatible to the ISDN basic rate. Such devices are ideal for providing the "U" interface point in an ISDN application.
Such devices are used in "smart" telephone instruments that have both a voice and data capability. Within such a telephone instrument the DNIC would terminate the line and encode/decode the data and voice for transmission. Additional electronics could provide interfaces for a standard telephone instrument and any number of data ports supporting standard data rates for such things as computer communications. One such device is the MT8972 DNIC, manufactured by the MITEL.RTM. Corporation.
The design of the analog interface between the DNIC and the transmission line is a difficult problem due to a concession between two issues. The first issue, is the design of a network of components, connected via a transformer, that provide the necessary near-end cancellation of the transmitted signal. The cancellation must be designed for transmission line loop lengths from zero to a large distance, and for 22, 24 and 26 gauge cable. The second issue, is the design of an intentional phase error in the near-end cancellation to meet a required one-half sampling clock within the DNIC. Thereby, allowing the DNIC to internally cancel correctly.
The solution to the above issues achieves a circuit that functions correctly for only moderate transmission line loop lengths, nonetheless, many customers require extended ranges for their particular applications.
In these cases, loop extender circuits are used to extend the operating range or the transmission line length over which the DNIC may operate efficiently. These loop extender circuits are found between the DNIC and the transmission line and basically combine signal equalization and amplification for the receive and transmit paths of the DNIC. One such device is the MH89726/728, manufactured by the MITEL.RTM. Corporation. However, a major problem with the loop extender circuits is that they can not be used for zero or near transmission line loop lengths. In the MICROELECTRONICS DIGITAL COMMUNICATIONS HANDBOOK, issue seven, by the MITEL.RTM. Corporation, application sheet MSAS-46, FIG. 2, page 15-11, suggests the use of three switches or jumpers to connect or bypass the loop extender circuit depending on the loop length to be achieved. The problem with this method is that, to correctly design a circuit to include the compromise issues discussed above would require also changing the electronic network components, that is, a clock correction to meet the one-half sampling clock requirement for long loops and no-clock correction for near loops.
Accordingly, it is an object of the present invention to provide an improved bi-directional buffer line amplifier that will perform efficiently with transmission lines that have loop lengths ranging from zero to a long range, with no data transmission errors.